The present invention is directed to hollow catalyst particles comprising a layered shell structure (also called “hollow spheres”) and to a method of their manufacture. The advanced hollow particles comprising a layered shell structure contain precious metals and find use in various catalytic applications, for example in gas-phase catalysis, in electrocatalysts for fuel cells or in catalytic converters for automobiles. They may also be useful in variety of other applications such as, e.g., in electronic or medical applications. The hollow spheres according to the present invention may be manufactured by forming a base metal core in the first step, depositing a precious metal onto said core in the second step, thus obtaining core/shell type particles, and removing the base metal core in the last step.
Electrocatalysts for fuel cells in automotive applications are generally based on nanoparticles (medium size <10 nm) of platinum alloys supported on high surface area supports (e.g. carbon blacks). To become commercially competitive it is still necessary to improve the activity of the platinum based catalysts even further than currently achieved with PtCo alloys.
In the present invention, the term “alloy” means a partial or complete metallic solid solution.
Core/shell-type catalyst particles such as, for example, Pt-coated Ni particles or Pt-coated Co particles gain increased importance and find use as catalysts for fuel cells or electrode materials for batteries. Especially the particles with a Pt-based shell reveal a high specific activity. As an advantage, they possess low precious metal contents due to the core/shell structure. The catalyst particles are characterized by a high specific mass activity (“SMA”) and an improved performance in oxygen reduction reactions (“ORR”) at the cathode of PEMFCs (Polymer electrolyte membrane fuel cells) or DMFCs (Direct methanol fuel cells). However, precious metal atoms which are not located at the surface of said core/shell particles are not accessible for catalytic reactions.
Hereinafter, the terms “precious metal” and “noble metal” are used synonymously.
J. Zhang et al. reported the preparation of core/shell particles as electrocatalysts for oxygen reduction. The core comprises of an alloy of a precious metal, whereas the shell consists of a Pt monolayer deposited by under potential deposition (“UPD”); ref to J. Zhang, F. H. B Lima et al, Journal of Physical Chemistry B Letters, 2005, 109, 22701-22704. The catalyst thus obtained is a PtMLX/C (X=Au, Ag, Pd; ML=monolayer) with the metal particles comprising an inner core consisting of metal X and a monolayer of platinum in form of a shell on top of it.
Core/shell catalysts comprising a ruthenium core coated with platinum were described some years ago (ref to S. R. Brankovitch, J. X. Wang and R. R. Adzic, Electrochemical and Solid State Letters 2001, 4, A217-A220). The medium particle size of the Ru/Pt core/shell particles is in the range of 2.5 nm (by TEM).
U.S. Pat. No. 7,053,021 B1 teaches the preparation of carbon-supported core/shell nano-particles of 1-3 nm size comprising a platinum-vanadium-iron alloy. An improvement by the factor of 2-4 is reported. Again, this improvement is still not sufficient to meet the targets of the automotive industry.
Examples for core/shell-type catalysts, primarily for use as electrocatalysts in fuel cells are disclosed in U.S. Pat. No. 8,227,372, U.S. Pat. No. 8,288,308, U.S. Pat. No. 8,304,362 and US 2012/0316054 to the same applicant. These core/shell particles comprise a metal or ceramic core material and at least three atomic layers of platinum in their shell.
The formation of base metal cores is often performed by reducing a base metal salt in a polyol. An example for the formation of Ni and Cu nanoparticles by the polyol method is described in K. J. Carroll, J. U. Reveles, M. D. Shultz, S. N. Khanna and E. E. Carpenter, J Phys Chem C 2011, 115, 2656-2664. Nickel occurs in two allotropes: the face-centered cubic (fcc) crystal structure and the hexagonal close packed (hcp) structure. Said paper describes the formation of fcc Ni nanoparticles. The synthesis and magnetic properties of fcc and hcp Ni nanoparticles through the polyol process are described in C. N. Chinnasamy, B. Jeyadevan, K. Shinoda and K. Thoji, J. Appl. Phys., 2005, 97, 10J309 and in K. S. Rao, T. Balaji, Y. Lingappa, M. R. R. Reddy and T. L. Prakash, Phase Transit 2012, 85, 235-243.
The formation of Ni@Pt core/shell nanoparticles is described in Y. Chen, Y. Dai, W. Wang, and S. Chen, J. Phys. Chem. C 2008, 112, 1645-1649: a Ni salt is reduced in a polyol, then H2PtCl6 is added dropwise to form a Pt monolayer shell. However, this method is not suitable for the manufacturing of multilayered core/shell particles comprising a base metal/precious metal alloy.
WO 2012/123442 A1, also published as US 2012/0238443 A1 to the same applicant is directed to a method for manufacture of metal nanoparticles, in particular to the manufacture of nano-sized base metal particles. The manufacturing method of this invention is based on the “seed particle method” or “seed-mediated method”. By this method, size-controlled base metal particles with a medium particle diameter in the range of 20 to 200 nm can be produced using small precious metal seed particles (“nuclei”) to initiate the particle formation. Such particles are preferably used as starting material in the present invention.
U.S. Pat. No. 7,211,126 B2 discloses a method for preparing non-magnetic nickel powders. Nickel with an fcc structure is ferromagnetic, whereas hcp nickel is non-magnetic. The fcc crystal structure is also known as cubic close packed (ccp); these are two different names for the same lattice. In U.S. Pat. No. 7,211,126 B2, a nickel precursor and a polyol are mixed and heated at 45° C. to 350° C. to form fcc nickel particles in the first step. In all embodiments of said disclosure, this first step is carried out at 190° C. for 10 min to 1 h. In a second step, the fcc particles obtained by the first step were heated at 190° C. for 24 h to form hcp nickel. By contrast to U.S. Pat. No. 7,211,126 B2, the inventors of the present invention found that the formation of fcc or hcp Ni depends on both the reaction time and the temperature. The higher the temperature, the less time is needed to form hcp Ni and vice versa. Furthermore, the inventors of the present invention found that the formation of hcp Ni by reducing Ni2+ salts in a polyol does not necessarily require a two-step process. If an appropriate combination of reaction time and temperature is chosen, a one-step process leads to hcp Ni likewise.
WO 2012/009467 A1 discloses hollow metal nanoparticles and methods for their manufacture, wherein the nanoparticles preferably comprise at least one noble metal. Said particles are manufactured by making solid core particles consisting of a less noble metal in a first step and adding a salt of a more noble metal in a second step. The less noble metal reduces the ions of the second, more noble metals. Since this method is quite slow, the formation of the noble metal shell is then continued by electrodeposition with an RDE setup. At the end of said process, the less noble metal has been dissolved in its entirety, and the remaining hollow particle consists of a discontinuous noble metal shell. The average particle diameter is 10 nm or less. By contrast to WO 2012/009467 A1, the present invention provides hollow spheres with average diameter of between 10 and 200 nm, preferable larger than 20 nm. Furthermore, the particles according to the present invention comprise a multi-layered shell that does not consist of pure precious metal, but comprises a base metal-precious metal alloy. The method for making said particles differs significantly from the method of WO 2012/009467 A1 as is does not comprise an electrodeposition step, and the base metal core is dissolved by acid leaching.
Leaching of solid solution metal alloys comprising a noble and a less noble metal to remove the less noble metal at least in part from the alloy is well known in the state of the art.
US 2011/0177432 A1 discloses a method of producing a porous metal comprising producing an alloy consisting essentially of platinum and nickel according to the formula PtxNi1-x wherein x is at least 0.01 and less than 0.3 and dealloying said alloy in a substantially pH neutral solution to reduce the amount of nickel in said alloy to produce said porous metal. The dealloying preferably occurs in a neutral electrolyte at potential greater than 2.1 V. According to this disclosure, dealloying in the NiPt system can also be done in acidic solutions, but the neutral solutions are benign.
EP 1 524 711 A2 discloses an electrode catalyst having a noble metal-containing particle deposited on an electroconductive carrier, wherein the noble metal-containing particle possesses a core-shell structure comprising a core part formed of a noble metal alloy and a shell part formed of a noble metal-containing layer different from the core part and formed on the periphery of the core part. The particle contains a noble metal and a transition metal. The method for the production of the catalyst comprises a step of depositing a noble metal-containing particle formed of a noble metal alloy on an electroconductive carrier and a step of exposing the surface of said particle to a solution capable of liquating a component other than noble metal, thereby forming a core-shell structure in said particle. The liquating agent is chosen from aqua regia, nitric acid, or concentrated sulfuric acid.
U.S. Pat. No. 7,223,493 B2 discloses a supported catalyst for a fuel cell containing a conductive support and a platinum alloy supported thereby. The catalyst is produced by first dispersing a support, e.g. a carbon support, in an aqueous solution of a platinum precursor, e.g. H2PtCl6, and then reducing the precursor to platinum. In the next step, a transition metal precursor is added and reduced to the metal. The transition metal preferably is iron, cobalt, nickel, copper, or manganese. Then, the particles are heated to form a platinum alloy on a conductive support. Finally, the particles are treated with an acid, e.g. aqueous sulfuric acid, to remove an unalloyed portion of the transition metal.
In summary, the electrocatalyst presently state of the art are not sufficient to meet the performance and cost requirements required for the widespread commercial introduction of fuel cell technology.
It is one objective of the present invention to provide electrocatalysts having an increased area of exposed catalytic surface area compared to typical core-shell catalysts with solid cores.
It is a further objective to provide a method for the preparation of such catalyst materials having an increased area of exposed catalytic surface area. The method should be based on a simple and economic synthesis route; it should be environmentally safe and should be easily scaleable for industrial production.